We have isolated two series of novel natural products from two Caribbean blue-green algae, Anabaina oscillarioides and Microcoleus lyngbyaceus. All of these secondary metabolites show good in vitro cytotoxicity and one, for which we have unambiguously assigned a novel styrylchromone structure, shows excellent cytotoxicity with ID50 values in the nanogram range. Further, this latter natural product selectively inhibits RNA synthesis in two cancerous cell lines. Our subsequent collections of these same two blue-greens from different habitats and seasons have pointed out the extreme chemical variability of these life forms. The extracts of these latter collections are strongly biologically active yet contain very different chemistry by TLC analysis than the previous collections. The above observations lead us to propose a systematic investigation of these and other Caribbean blue green algae for new cytotoxins. Further, we plan to explore the influence of environmental factors on blue green algal secondary metabolism by seasonal collections and labaoratory culture work. Our long term objectives are to discover new chemotherapeutic agents for treating neoplastic diseases, and hence, to further understand the structural requirements for cellular toxicity.